For proper function, steroid receptors must form complexes with hsp90 and several co-chaperones that bind to the hsp90 C-terminus via TPR domains. The roles of these co-chaperones are poorly understood, but they are likely to regulate receptor localization and binding affinity. We have found differences between TPR, hsp90 binding sites that might be exploited to develop specific inhibitors. The broad, long-term objectives of this proposal are to understand the roles of specific hsp90 co-chaperones in regulating signaling by steroid receptors, and to develop more specific hsp90-targeted drugs than are currently available. Our specific aims are: 1. To determine the roles played by TPR-containing co-chaperones in the glucocorticoid receptor (GR) pathway. We hypothesize that different co-chaperones have some redundant functions but that they also have specific functions resulting from their structural differences. 2. To compare the regulation of different steroid receptors by different TPR-containing hsp90 co-chaperones in living cells. Different steroid receptors associate preferentially with different co-chaperones. We hypothesize that this preferential association is required for normal responses to steroid hormones. 3. To identify peptide inhibitors that specifically block the binding of particular TPR-containing co-chaperones to hsp90. We hypothesize that inhibitors of the binding of particular TPR proteins to hsp90 will differentially alter signaling by different steroid receptors. The health relatedness of the project lies in gaining a better understanding of proteins important in cancer and endocrine disorders, and in obtaining information that could lead to the development of more specific hsp90-targeted drugs than are currently available. The experimental plan is: 1. To use RNAi to test the role of hsp90 co-chaperones in GR nucleocytoplasmic shuttling, formation of hsp90 heterocomplexes, binding affinity, and reporter gene activation. 2. To use RNAi to assess the role of hsp90 co-chaperones in the functions of other steroid receptors. 3. To screen phage display libraries for peptides that specifically block binding of particular co-chaperones to hsp90. To use TAT-fusions and microinjection to test these peptides' effects on steroid receptor function in living cells.